I was always led to believe that the Flood occurred to rid the Earth of, not only the wicked people, but also the Nephilim (I wonder if Noah ever preached to them......and what if just ONE of them became a dub...sorry, off on a tangent).
Clearly God created Angels with the ability to take on human form and procreate......fast forward to 1914. Prior to 1914, Satan and His angels had free passage between the Heavens and Earth. In 1914, Christ took His (invisible)Throne in Heaven and cast out Satan and His followers (hence the disruption on Earth - WWI), and Satan is now doing as much damage as possible on Earth where he is resident.
It is reasonable to conclude (couldn't resist, under_believer) that God did not take away the ABILITY for angels to do this (unless I missed it in the Bible), So WHY then are we not seeing a new breed of Nephilim?